In many industrial processes, surface-active substances are employed in order to obtain desired effects. For instance, aqueous coating compositions require a range of auxiliary products and additives, examples including emulsifiers needed to emulsify the water-insoluble binders, or additives to improve substrate wetting, and pigment dispersion. An undesirable side effect of these surface-active substances, however, is that they stabilise, in the form of foam, air which may be introduced in the course of preparation or application of the said coating compositions.
Use of organic silicon compounds in the form of oils, especially of dimethylpolysiloxanes of low to medium viscosity, as an additive for defoaming aqueous solutions or dispersions has been known, and has been described, for example, in the book by W. Noll “Chemie and Technologie der Silicone”, Weinheim 1968 [Chemistry and Technology of Silicones]. The defoaming activity of antifoaming agents based on organic silicon compounds, and also of defoamers based on mineral oil can be improved by adding highly disperse inorganic or organic substances, especially the so-called pyrogenic silica grades as described in DE 10 67 003 B and DE 1 914 684 C3. Defoamers based on polyoxyalkylene-polysiloxane copolymers have ben described in U.S. Pat. No. 3,763,021. While these defoamer additives are suitable to suppress or reduce the propensity to foaming in polymer dispersions or aqueous coating compositions, aqueous coating compositions comprising these additives exhibit also disadvantages which are attributable to the defoamer additives. High-gloss emulsion paint systems to which polysiloxanes, polyoxyalkylene-polysiloxane copolymers or formulations based on mineral oil have been added to suppress or reduce formation of foam have shown wetting defects, and also have reduced gloss when applied to surfaces. Wetting of the substrate is not uniform across the area covered which leads to the formation of coating films of varied thickness and defective zones in the coating film. There are also problems of interlayer adhesion associated with the use of defoamers based on organic silicon compounds, craters appear in the coating films particularly if the coating compositions are applied by dipping the substrate into a tank filled with a coating composition. If volatile compounds evolved during drying and stoving are fed to catalytic combustion units as is habitually in industrial coating, the service life of the catalysts is reduced by the silicon content.
Defoamers based on mineral oil are prone to reduce the gloss of emulsion paints, and lead, in flexographic printing inks, to unwanted swelling of the flexographic plates.
In DE 36 36 086 A1, fatty acid esters of polyglycerol polyglycol ethers are disclosed that are obtained by conventional reaction of polyglycerols with ethylene oxide, propylene oxide or their mixtures, and esterifying the resulting alkylene oxide adduct with fatty acids. Foam prevention properties of these products still need to be improved with regard to the limited service time of these compounds in the paint.
None of the systems known heretofore, however, provides satisfactory results for such coating compositions that are applied by the so-called airless spray technology.
As is known, the airless spray technology does not use compressed air to transport the coating composition through the spray nozzle, and to form small droplets of paint. Instead, paint is pumped at increased fluid pressures (3.4 MPa to 44.8 MPa, 500 psi to 6500 psi) through a small opening at the tip of the spray gun to achieve formation of small droplets, referred to as “atomisation” by the person skilled in the art. Pressure is generally supplied to the gun by an air-driven reciprocating fluid pump. When the pressurised paint enters the low pressure region in front of the gun, the sudden drop in pressure causes the paint to “atomise”. Airless systems are most widely used by painting contractors and maintenance painters. Airless spraying has several distinct advantages over air spray methods. This method is more efficient than the air spray because the airless spray is softer and less turbulent, thus less paint is lost in bounce back. The droplets formed are generally larger than conventional spray guns and produce a heavier paint coat in a single pass. This system is also more easily portable. Production rates are nearly double, and transfer efficiencies are usually greater (ranging from 65% to 70% of the paint used). Other advantages include the ability to utilise high-viscosity coatings (without the need of adding solvents to reduce the viscosity) and its ability to have good penetration in recessed areas of a workpiece. The major disadvantage of the airless spray is that due to the thick layer buildup and high viscosity of the airless coating formulations the resulting films are very sensitive to form pinholes and air entrapments and the demands for defoaming reagents are very high.